My .02 cents...
Mileage varies based on so many things. Location, truck mods, truck condition, driver variables, etc.
Location. Driving through the mountains, like when I drive from New Mexico to Utah. I fill up half way. And in every single vehicle I have driven thru that route, the second tank is ALWAYS better. Because When I drive thru the mountains, what goes up MUST come down. Whatever horrific mileage I get climbing that mountain more than makes up on the other side. It's no different than in an airplane. I'm a pilot. If we're trying to save gas, we always try to get as high as possible, so we can "make gas" on the descent. However, driving I-10 thru Texas, the constant up and down eats the mileage bigtime. The climbs are just enough to rob the fuel economy, and the descents just aren't long enough to recoup the loss. I've noticed this on ALL the vehicles I've driven on that freeway. Arkansas is rolling hills, and this might have something to do with it.
Truck mods. If you lift/level the truck, you're gonna lose a little. I have an ARE topper, and the weight and the aero interference affects the mileage negatively. Larger tires, gonna take away mileage. If the truck isn't running in top shape, it's going to affect mileage. Your gears are gonna affect mileage as well. 3.92's will be better around town than 3.21s, and 3.92s will be better for moderate highway speeds. Anything over 65 or 70, and your RPMs are higher, thus getting worse mileage. However, 3.21s don't get any better, I have found, at higher speeds. What you might "gain" due to lower RPMs at speeds will be negated by the higher wind resistance. It is what it is.
Driver variables. Well, we all drive different. 'Nuff said on that.
My truck. It's a 4x4 5.7L Laramie. Front is leveled. Slightly bigger-than-stock E-rated tires. 275 65 20's. I have a tuner, bigger throttle body, headers, high-flow cats with new O2 sensors, Gibson cat-back exhaust. 6.4L Hemi Manifold with SnB intake. Only reason for the SnB intake over stock is because the 6.4 intake manifold routes the intake hose different. And prolly a couple other mods I'm forgetting.
With the tune, I run slightly aggressive throttle response, but a fuel-mileage tune. I'm corrected for tire size. First and foremost, I love this truck! I love the sound with the exhaust mods. The tune and all the other small mods add to a little more power and make it a joy to drive. Second, it's a truck. I would love to get 25 mpg, but that's a pipe dream. It's a truck. However, if I was getting 12 mpg on a regular basis, I too would be concerned. If I had my foot in the throttle constantly, my joy would compensate for the frown from the mileage. But if I was trying to get better, then I would try to figure out why.
With my tune, running the fuel economy mode doesn't do much for daily driving, or when empty. It did, however, bump up my towing mileage by 2-4 mpg. No joke. I was getting 10-11 towing, and now, with the same trailer, same load, 12-14. And this is alway HAND CALCULATED.
A couple of things I regularly do if I notice mileage going down, I try to drive mellow on the next tank to see if I was driving a little spirited on the last tank lol. I check the tire pressure. E-rated tires, I run 60-65 psi. This will help the mileage, and the mileage of the tires. BTW, I've had these Cooper Discoverer ATP's for 80k miles. Yes, 80k. Put them on at 17k, getting rid of the crappy "BadYears" and have almost 100k on the truck now. Still an eighth of an inch from the wear bars. Keep the truck tuned. Check/replace the plugs. Either take out the injectors and clean them, but what I would do first is get some Sea Foam and put in the tank. Recently, I took two cans of Sea Foam and put in the oil and the fuel tank. 1 oz per quart of oil in the crankcase and 1 oz per gallon in the tank. So, I bought two 16 oz cans. Put about half of one can in the engine oil, and the rest in the tank. I did this before the last oil change. If you've not heard of this, you can look it up. Putting Sea Foam in your oil 100-300 miles prior to an oil change. Anyways, then I drove about 150 miles over a couple of days and changed the oil. Used Royal Purple 5w20. I'm getting 1-2 mpg better. The first tank, I was like "No way." The next tank, same thing. The next tank, I was like, "Let's see if I can do better." Started driving really mellow. No JackRabbit starts, kept my speed under 70 on the highway, etc. And my normal mpg is 16-17. I was getting 18-20. Hand Calculated. So, check your tire pressure, use some Sea Foam and use good oil, do what someone mentioned and put a bottle of something in the back and see if you can drive without it falling over, and see what happens. Oh, and what others have said, your mileage will go up in a couple of months anyway when the winter blend is gone.
Running around in 4auto the last week or so in the snow... last fill up mine got 8.9mpg. But that’s a lot of idling and hauling wood to folks. Pretty decent for 199k mile 2008 model 5.7 I reckon.
